To specify a budget in a Microsoft Advertising campaign, you set a daily spend target. Microsoft Advertising uses this daily target to calculate a budget for a full month. While your actual spend for a specific day might be higher or lower than the target you specified, the overall spend for a month will not exceed the monthly amount Microsoft Advertising calculated for you.
To prevent you from overspending your budget, Microsoft Advertising automatically pauses your campaigns when you reach the amount your overall budget allows in a given day. Microsoft Advertising will resume your campaigns the next day.
Note that Search Ads 360 itself does not use the budget setting to manage your spend. Instead, the setting is posted to Microsoft Advertising, and Microsoft Advertising uses the setting to make sure that your spend is spread out over an entire month.